1. Field of the Invention
The invention relates generally to sputter deposition of materials. In particular, the invention relates to a shield used in a sputter reactor.
2. Background Art
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and metal nitrides in the fabrication of silicon integrated circuits. Recently developed technology has enabled sputtering to be applied to many difficult structures, such as depositing thin barrier layers in high aspect-ratio holes.
In U.S. Pat. No. 6,296,747, Tanaka describes one such advanced plasma sputter reactor 10, illustrated schematically in the cross-sectional view of FIG. 1. The reactor 10 includes an aluminum reactor body 12 defining a vacuum chamber. A metal target 14 is supported on the wall 12 through an isolator 16 and faces a wafer 18 to be sputter coated. A wafer clamp 20 holds the wafer 18 on a pedestal electrode 22. A vacuum pump system 26 connected to the chamber through a pumping port 28 is capable of maintaining the interior of the chamber at a very low pressure of down to about 10xe2x88x928 Torr. However, a sputter working gas, such as argon is supplied from a gas source 30 and metered by a mass flow controller 32 to flow through an inlet 34 into the chamber at a pressure typically in the low milliTorr range. When a DC power supply 36 applies a negative voltage of about xe2x88x92600 VDC to the metal target 14, the argon working gas is excited into a plasma and the positively charged argon ions are attracted to the target 14 at high energy and sputter metal atoms from the target 14. Some of these metal ions strike the wafer 18 and are deposited in a thin layer thereon.
The reactor 10 is configured for self-ionized plasma (SIP) sputtering. A small magnetron 40 is positioned at the back of the target 14 and includes an inner magnetic pole 42 of one magnetic polarity surrounded by an outer magnetic pole 44 of the opposite polarity and of a substantially greater total magnetic intensity. The poles 42, 44 are supported on and magnetically coupled by a magnetic yoke 46, which is itself supported on a motor driven shaft 48 positioned along a center axis 50 of the chamber so that the magnetron 40 is rotated about the center axis. The magnetron 40 creates a magnetic field adjacent the interior face of the target 14 and thereby creates a region of high-density plasma next to the target 14, which intensifies the sputtering rate in the portion of the target 14 adjacent the high-density plasma. The magnetron rotation produces a more uniform sputtering pattern.
The sputtering process not only coats the wafer 18 with the sputtered metal atoms, it also coats any other body exposed to the target 14, such as the chamber wall 12. Cleaning sputtered material from the interior of the chamber wall 12 is difficult and time consuming. Accordingly, it is standard practice to include sputter shields, such as the illustrated upper and lower shields 54, 56, typically formed of aluminum or stainless steel, which protect the chamber wall 12 from sputter deposition and are instead themselves coated with the sputtered material. A topmost shield 58 protects the isolator and is positioned close to the target 14 to form a plasma dark space between it and target 14. When the shields 54, 56, 58 become excessively coated to the point that the coating tends to flake and produce deleterious particles, they are replaced with fresh shields in a preventative maintenance (PM) procedure. At least the lower shield 54 is usually electrically grounded to act as the anode in opposition to the target cathode in the plasma generation process.
In SIP sputtering, the magnetic field and the target power are increased to the extent that a large fraction of the sputtered metal atoms are ionized to produce two effects. First, the metal ions are themselves attracted back to the target to sputter yet further metal atoms in a process referred to as self-sputtering. As a result, the argon supply and chamber pressure can be decreased. In the case of copper sputtering, it is possible to stop the supply of argon once the plasma has been ignited. Secondly, the metal ions can be accelerated towards the wafer by an RF power supply 60 connected to the pedestal electrode 22, which results in a negative DC self bias on the wafer 18. A controller 62 controls the power supplies 36, 60 and the flow of gas to set the processing conditions. Further, the magnetic field created by the unbalanced intensities of the magnetic poles 42, 44 can guide the metal ions to the wafer 18.
Such ionized sputtering and controlled directionality is advantageous for sputtering material into deep and narrow holes, that is, holes having a high aspect-ratio. Aspect ratios of 5:1 are becoming common for inter-level electrical vias through silica-based dielectric layers, and aspect ratios are increasing for advanced products. As a result, sputtering can be used to deposit thin liner layers on the bottom and sidewalls of the via holes. One such liner layer is a barrier layer required to be interposed between the metal filled into the via and the silica dielectric to prevent the metal from diffusing into the silica and making it conductive. For aluminum metallization, a titanium-based barrier of Ti/TiN is typically used, where TiN is titanium nitride. For copper metallization, a tantalum-based barrier of Ta/TaN is more typical, although other barrier materials are possible. The titanium or tantalum is usually deposited first to act as a glue layer to the underlying silica while the nitride acts as the actual barrier material.
Sputtering, particularly ionized sputtering, can be used to deposit both the metal portion and the metal nitride portion of the barrier. The target 14 has at least its front surface composed of the metal, whether it be titanium, tantalum, or other barrier metals such as tungsten. In a first phase, called metal sputtering, a thin metal layer is deposited on the wafer 18 under biasing conditions that favor sidewall deposition. In a second phase, called reactive sputtering, nitrogen is additionally admitted into the chamber from a nitrogen source 66 through its mass flow controller 68. The nitrogen reacts with the metal atoms at or near the surface of the wafer to deposit a coating of metal nitride.
Reactive sputtering increases the problems associated with the sputtering shields. A first problem addressed by Tanaka is that nitrogen, unlike argon, is consumed in reactive sputtering. The gas inlet 34 is preferably located behind the shields 54, 56, and the gas flows into the main processing area through a gap 70 between the lower shield 56 the clamp 20, and the pedestal electrode 22. While this flow pattern is sufficient for argon, it constrains the supply of nitrogen and it is possible that the nitriding is incomplete. Accordingly, Tanaka forms a ring of perforations 74 in the lower shield 12 to facilitate the flow of nitrogen into the main processing region. However, to protect the chamber wall in back of the perforations, he additionally includes the upper shield 54 to cover the perforations 74 with a downwardly facing gap 76 between the two shields 54, 56 to flow the nitrogen from the perforations 74 into the main processing region. While the structure is effective, it is complicated.
Particulate flaking of shields in reactive sputtering is a particularly troublesome effect. Nitrides tend to be much harder and less pliable than metals Accordingly, they are more prone to flaking at lesser coating thicknesses. It has become typical to roughen the shield surface exposed to sputter coating. Machined grooves are disclosed by Koyama et al. in U.S. Pat. No. 5,837,057 and by Visser in U.S. Pat. No. 6,059,938. A more commercially popular approach is to form the shield of stainless steel and to coat the areas of the shield exposed to sputter coating with an aluminum layer applied by arc spraying. The so applied aluminum is very rough and improves the anchorage of the nitride film to the shield. Another technique, called pasting, involves the periodic sputter deposition of a relatively thick layer of the more pliable metal over the coated shield to paste the flake-prone nitride to the shield. Pasting is somewhat effective, but it interrupts the production scheduling.
Even with these techniques, shields used in reactive sputtering tend to begin flaking after a number of wafer cycles typical of the process. Accordingly, it is standard practice to replace the shields after a set number of wafer cycles in a preventative maintenance process. A typical number of wafer cycles between shield replacements is about 5000 wafers. This number compares to about 20,000 wafers for a barrier target after which the target needs to be replaced. The additional preventative maintenance steps required for shield replacement significantly reduce the throughput of expensive sputtering equipment. It is greatly desired to increase the number of wafer cycles before a shield needs to be replaced, preferably a number at least equal to the life of the target.
The roughened shields described above also tend to be relatively expensive. Accordingly, it is typical to refurbish a coated shield and again use it. For example, the nitride coating is stripped, for example, by removing the arc-sprayed aluminum then again arc spraying aluminum onto the affected areas. Refurbishment is typically done in speciality shops separate from the fab line and introduces logistical problems in shipping and control of used parts. It is thus desired to provide a shield that is inexpensive enough and has a long enough lifetime that it can be treated as a consumable, throw-away item without the need to be refurbished.
A shield used to protect chamber walls and other parts in a sputter reactor is composed of at least one layer of mesh. The mesh includes apertures having sizes in the range of 1 to 6 mm and a porosity range of 10 to 95%. Preferably, the mesh may have a regular two-dimensional pattern of apertures.
Multiple layers of mesh are laminated together. The meshes of the different layers may be of different grades, preferably with the mesh layer farthest from the chamber interior having the finest mesh. A foil may be interposed between two mesh layers, preferably the two farthest layers. If needed, the foil may be perforated to increase the gas flow through the mesh assembly.
The one or more mesh layers can be bent to a desired shape. Metallic hemming may be crimped or otherwise attached to the exposed ends. Holes may be cut in the mesh assembly and may be optionally lined with a grommet or similar circumferential cover.
The mesh may be formed, for example, of expanded metal mesh, woven wire, or crushed honeycomb.